In an image recording apparatus which opto-electrically reads density information of an image on an original by a CCD line sensor, and outputs the information in the form of an image, improvements of an image quality and an operation speed have been main subjects. Particularly, in case that the original is a color original, an imaging unit of the image recording apparatus is moved for scan four times, to read color signals of four colors. In this case, how to minimize color displacements among the four colors is of significance. To this end, it is necessary to minimize variation of the stop position of the imaging unit, variation of the time taken for the imaging unit to travel from a home position to a registration position, and variation of the scan speed. In the control of those variations, a stepping motor is superior to a DC servo motor. For this reason, a drive system for driving the imaging unit by the stepping motor has been developed and known.
The image recording apparatus alternately repeats two types of scan modes, a forward scan mode to scan an original by the imaging unit, and a back scan mode to return the imaging unit to an the original position. To realize a high speed operation of the image recording apparatus, it is desirable to return the imaging unit as quickly as possible. Accordingly, a speed profile of the imaging unit is designed such that in the forward scan mode, a constant speed region is assigned to an original read region, the imaging unit is accelerated and decelerated at fixed rates at the front and the tail edges of the original read region, and in the back scan mode, speed of the imaging unit is increased up to a much higher speed than in the forward scan mode.
The stepping motor has such a characteristic that, as the frequency becomes smaller and the feed current become larger, the torque becomes larger. The drive current of the stepping motor is selected to be a large value so that it provides a satisfactory torque in the back scan mode to return the imaging unit at a high speed, with such a safety factor as to prevent a step-out of the motor.
The stepping motor generates more vibration and noise than the DC servo motor. As already described, in the stepping motor, the torque is larger, as the drive frequency is lower and the feed current is larger. In a stationary state where the imaging unit travels at a low speed, the generated torque is excessive, thus creating many problems such as vibration, noise, irregular speed of rotations, and overheat of the motor. In the image recording apparatus based on four scans, the vibration causes fine displacement of colors and distortion of figure, and the like in the image as read. Accordingly, a quality of the read image is greatly deteriorated. When such color image data of four colors, Y, M, C, and K are combined to reproduce the original color image, the adverse effect by the vibration appears in the form of color displacements on the reproduced color image.